Electric discharge device



- .Oct. 2, 1956 T. E. FOULKE 2,765,417

ELEQTRIC DISCHARGE DEVICE Filed Oct. 16, 1948 /0/V/ZA BLE GAS FILLING Invervtov: Ted E. FouLke,

b 0W6. wM AM His A ti'ovneg.

Unied States tent" ELECTRIC DISCHARGE DEVICE Ted E. Foulke, Chagrin Falls, Ohio, assignor to General Electric Company, a corporation of New York.

Application October 16', 1948, Serial No. 54,873

4 Claims. (Cl. 313-44) My-inventionnrelates. to electric discharge devices, and more. particularly to glow discharge. or cold cathode devices having precise and uniform time-breakdown characteristics upon the application of voltage across terminals thereof, even though the devices are enclosed in containerswhich are impervious to light and substantially all other radiation.

It has been appreciated heretofore that when an electric discharge device is employed as a circuit controlling element having a desired predetermined time-breakdown or time-ionization characteristic it is desirable, and in many instances necessary, to construct the device so that a discharge is established in an accurately determinable interval of time after the application of voltage. across the terminals or across the. electrodes of the device. Short-time response characteristics and uniformity of ionization time, even though light or other ambient radiation is not present or available, are necessary in many applications. One of such applications is that where the discharge devices are totally enclosed in containers which are impervious to light and other radiation. Conventional present-day glow devices are so slow in response when shielded that they cannot reliably be controlled. in less than about 2 seconds. I

Generally speaking, it may be said that the time-breakdown, or ionization characteristic, of glow discharge devices is av function of the number of electrons available to. be. acted upon by the voltage impressed between cooperating electrodes, and which upon acceleration by the resulting electric field obtain a sufiicient velocity to. ionize the medium, vapor or gas, through which the discharge is to pass. Of. course, these. electrons may be furnished by thermionic emission, but such thermionic electrodes are. not always satisfactory for all applications, particular- 1y when there are limitations; as. to auxiliary Sources of power, or as to the desirability for long life, and: permanency. Alternatively, in order to assure a sufficient number of electrons for ionization purposes, it has. been proposed heretofore to place means for this purpose with in. the enclosing envelope or bulb of the device, but such arrangements have been found unsatisfactory because of the contamination produced inside the envelope and because of variations in the electrical characteristics produced thereby.

It is an object of my invention to provide a new and improved electric discharge device.

It is another object of my invention to provide a new and improved glow discharge device which has. an ac- .curate predetermined maximum time-breakdown characteristic of the order of 10- to 10" that of the present commercial devices even whenemployed in the dark, orbeing totally enclosed and shielded from ambient radiati-on.

It is a further object of my invention to provide a new and improved electric discharge device which may be employed as a lightning arrestor.

V It is a still further object of my invention to provide a new and improved electric discharge device having a 5 ice maximum breakdown or ionization time of the order'of one microsecond, even in the absence of light or other radiation.

Briefly stated, in accordance with the illustrated embodiment of my invention, I provide an electric discharge device, suchas a glow discharge device of the cold cathode type, in which cooperating electrodes are placed within an enclosing envelope. At least one of such electrodes is; coated with an activating composition or coating of oxides of the alkaline earth metals. Outside the enclosure definedby the envelope, I provide a material of many years duration for producing radiation which causes the photo-emission of electrons in sufficient numbers to as sure a maximum time of ionization or breakdown of the discharge path upon application of voltage across the electrodes thereof. The envelope is pervious to such radiation, and preferably transmissive of radiation with in the. ultraviolet region down to about 3200 to 3300 Angstrom. Units. The spectra of such radiation is cor related to the electrode surface so that a substantial part of the energy thereof is of a wavelength less than the threshold wavelength of such electrode. surface.

For a.. better understanding of my invention reference maybe had tothe following description taken in connectionwith the accompanying drawing and its scope will be pointed out, in the appended claims. The single figure of the drawing illustrates an embodiment of my inven tion. a applied. to a. glow discharge device which is totally enclosed in a container impervious to light, and other; radiation.

Prior to; considering the embodiment of myinvention shown in the single figure of the drawing it. is believed desirable to consider generally the fundamental relation: ships-which exist in producing photo-emission of-electrons from ametallic surface, or a surface which is provided with an. activating coating. Generally, it may be stated that where the, frequency of the radiation is greater than that; corresponding to the critical or threshold frequency establishedby the chemical and. physical properties; of the surface irradiated, electrons will be emitted photo-electrically, at a velocity which is determined by the; quanta r lation hip V0=equivalent imparted energy in volts. e=charge of the electron.

m=mass of the electron.

v=velocity.

h=universal constant.

v=frequency.

- The. relationship between the work function a and the critical or thereshold frequency we is established by:

V=emi-tted velocity in volts. 7 p=work function of the surface involved (in volts) vo.=f,requency corresponding to'the long-wavelength limit, that is, the threshold frequency huo=rninimum energy lost by an electron escaping from the. surface.

If the quantity expressed by Equation 2 above is negaf tive, no electrons will be emitted, or escape from the surface. This condition is that which exists when the quanta of energy is not greater than the energy'corresponding to the threshold frequency.

Referring now particularly to the single figure of the drawing, I have there illustrated my invention as applied to a glow electric discharge device 1 which is totally enclosed and shielded by a container 2 which isconstructed of a tunable metal such as brass, and which is impervious to light and other radiation. The glow ,di$i..'.

charge device comprisesangenclosing'envelope 3' constructed of glass or quartz which is pervious and transparent to radi'ationwithin theblue and violet-.blucpor'.

I tion of the visible spectrum and is also permeable. to

radiation within the ultraviolet region of the spectra, par-.. ticularly that'lying above approximately 3200 Angstrom Units. The transmittingcharacteristics of the material employed as the envelope 3 will of coursebe, determined by the radiation desired for exciting the electrode sur- 5 which may be'constructed of nickel and which are, supported by sealing structures 6 and 7 comprising, part of: a press or header 8 and through'which extend lead-inconductorsi 9 and 10 constituting, externally accessible terminals. The electrodes 4 and 5 preferably are long nickel cylinders supported by lead-in conductors 9 and 10.

i Either one or both of-theelectrodes '4 and 5 may be. provided with an activating coating '11 which is pref erably formed by the addition of oxides of the alkaline earth metals to the conductor: material constituting the electrodes. i Preferably, I mayemploy oxides of barium and strontium to obtain a work function' thereof less than 2 volts and preferably less than 1.5 volts.

i Within the envelope 3 .I employ a suitable 'ionizable medium, such as a rare gas from the group consisting i of argon, neon, krypton and xenon and mixtures thereof;

at: a suitable pressure depending upon the characteristics 'of the discharge device desired. For example, I have found that'a gaseous composition comprising neon with i about ofi'per cent argon at a pressure ran'gng from I to .60 mm. of mercury has operated very satisfactorily in adevice'required to operate at 8'5 volts-11C. To ob-' tain' devices operable at'other voltages either pressure or gas composition may be altered over wide limits.- 'As a l I means for irradiating the electrode surfaces of either, or

the.- radiationfor efiect ns p o o-emis ion of el c r ns from surface llgmay be a; radio-active phosphor paint I applied to the outsidesurface, of the envelope 3. A1-

T though'not limited thereto, one such paint may comprise faces described hereinafter. Iprovide electrodes 4 and V a phosphor orfluorescent, material comprising zinc sul I phide activatedwith silver and'a radio-active material such as radium-sulphate Iwhich excites the fluorescent material or phosphor which in turn produces the radiation which is transmitted through the envelope 3 and impinges I i upon the surface 11. I

'I have found in choosing arnaterial for this purpose that materials emitting within the blue region areIvery I i Y I effective.- Using a phosphor with maximum emission be- I=providecaesium .as, an ingredient of the gaseous corn position,;and caesium may be added to the dischargeflin I the form of caesium dichromate which upon. evaporation :during processing, ofthe device willof course settle in some 'measure. upon the activating surface 11. 1 .I have found that where caesium is employed; the maximum photo-electric current or the maximum photo-emission I I appears to occur at about 4{300-Angstrom units, whereas I where oxidesIofjbarium'and Istrontum are employ ed as I theactivating surface 11 the maxmurn photo-emission I appears to occur within the range; from about 3200 to I both, electrodes 4 and 5 by radiation correlated in frequency (and wavelength) to the threshold frequency (and I 1 I I wavelength) of the electrode surfaces, I provide material '12 which ispositione'd outside the enclosure provided by I the envelope 3, and which may preferably be placed upon the exterior surface of the envelope 3. In this manner no contaminating products are produced inside the envelope 3 and therefore the electrical and operating characteristics of the glow discharge device 1 are not modified or changed.

The frequency of the radiation produced by the material 12 is preferably sufiiciently greater than the critical or threshold frequency of the electrode surface 11 so that I electrons are photo-emitted at a sutficient velocity to be present in the ionizable medium at the time voltage is impressed across the electrodes 4 and 5. The quantity of, radiation produced by the material 12, and accordingly the intensity of the radiation impinging upon the activating surface 11 of electrode 4, is sufliciently high so that there are enough electrons in the ionizable medium at the time the voltage is impressed across electrodes 4 and 5 so that the ionization time is not only uniform but very much smaller than that previously obtainable in discharge devices of this nature. Heretofore, the time-breakdown characteristic or the ionization time has been in the order of magnitude of seconds or a substantial fraction thereof when completely shielded, whereas in the presence of light it is of the order of a few micro-seconds, and these characteristics and ionization times have not been uniform in any one type of device or in any one device. By employing my invention, I have been able to obtain not only uniform time-breakdown characteristics and ionization times, but I have also'been able to reduce the shielded ionization time from the order of a second or more, to 1000 to 1 microseconds, dependent only on quantity and quality of radiation. ,I

A suitable meansand material for use in supplying ell) - 4000 Angstrom units, 'and'more' particularly between 3300 "and 3800 Angstrom units.

course the ionizable medium is arare, gas, or mixture of rare gases]. i

I I Inasmuch as; uniform: electrical characteristics of the discharge device 1 are highly important andvital, I; have I provided means for preventing: electricalleakagebetween I electrodes 4 and 5 and inleads 9 10,. which in some instances may be occasioned by the use of caesium which settles on the stems and seal structures. This leakage prevention expedient comprises placing oxides of light metals, such as oxides of calcium and magnesium, on the supporting structures or seals 6 and 7 to repel and prevent deposition of caesium thereon. Light metal oxides may also perform a function in addition to the prevention of electricalleakage, and that is the emission of secondary high speed electrons from the electrode surfaces by radioactive bombardment by substances, such as thoria, which do not poison the gas, or iouizablemedium, and which do not attack the electrode surfaces. I

-Where it is desirable to provide within the envelope 3 materials which cause secondary particle emission from material placed on the elements 6 and 7, I may employ the following composition on the supporting stmctures 6 and 7, which produce alpha, beta and gamma rays to assist in the ionization of the medium employed and hence to provide and have present a small but permanent and additive source of ionization:

grams of thorium oxide 10 grams of magnesium oxide 10 grams of calcium oxide 10 grams of beryllium oxide 10 grams of aluminum oxide By employing the above-described features in the construction of glow discharge devices I have been able to reduce the shielded ionization timefrom seconds to sew era uq a it m oec n n in e mates In the latter instance, of I to 0.1 micro-second. Discharge devices constructed in accordance with the teachings of my invention not only have afforded reduction in the ionization and response times but have also afiorded uniformity of response characteristics without deterioration of other electrical properties.

Electric discharge devices constituting my invention may be employed for several purposes such as in lightning arresting devices, and particularly in any electrical circuit requiring reliable response Within a predetermined maximum interval of time ranging to values as small as one micro-second and less. These time-response characteristics are obtained without sacrificing the electrical characteristics, such as the breakdown voltage and the discharge sustaining voltage which of course, vary depending upon the design of each device incorporating my invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. A glow discharge device having a precise timebreakdown characteristic upon application of voltage thereto, comprising an enclosing envelope permeable to radiation having a wavelength between about 3200 and 4900 Angstrom units, a pair of cooperating electrodes in said envelope at least one of said electrodes being a cold cathode having an activating coating of oxides of the alkaline earth metals, sealing and supporting structure for said electrodes, an ionizable medium within the envelope comprising a gas from the group consisting of the rare gases and caesium, material positioned externally of and contiguous said envelope and producing radiation having wavelength between about 3200 and 4900 Angstrom units for efiecting photo-emission of electrons from said one electrode, and oxide coating on said structure of light metal which repels cesium for preventing leakage between said electrodes.

2. An electric discharge device comprising an enclosing envelope, a pair of cooperating electrodes in said envelope at least one of said electrodes being a cold cathode having an activating composition thereon comprising an oxide of the alkaline earth metals, an ionizable medium within said envelope comprising a rare gas and caesium, radio-active fluorescent material positioned externally of and contiguous said envelope and producing radiation which efiects the photo-emission of electrons from said one electrode in order to ionize said medium,

and an oxide of the group of metals consisting of magnesium, calcium and mixtures thereof upon the sealing and supporting structure for said electrodes to prevent the deposition of caesium thereon.

3. An electric discharge device comprising an enclosing envelope, a pair of co-operating electrodes in said envelope at least one of said electrodes being a cold cathode having an activating composition thereon comprising an oxide of the alkaline earth metals, an ionizable medium within said envelope comprising a rare gas, material positioned externally of and contiguous said envelope and producing radiation which efiects the photo-emission of electrons from said one electrode in order to ionize said medium, upon application of voltage, and a composition positioned on the sealing and supporting structure for said electrodes comprising an oxide of the group of metals consisting of thorium, magnesium, beryllium, aluminum and mixtures thereof for producing rays to assist in the ionization of said medium by bombarding said medium to thereby cause additional ionization.

4. A glow discharge device having a precise time-breakdown characteristic upon application of voltage thereto comprising in combination an enclosing envelope pervious to blue and ultraviolet radiation, a pair of cooperating electrodes in said envelope at least one of which is a cold cathode and has thereon an activating composition having a work function less than two volts, a rare gas in said envelope, and phosphor of radioactive silver-activated zinc sulphide positioned externally of and contiguous said envelope and emitting radiation which efiects photo-emission of electrons from said one electrode, the combination imparting to the device a maximum time-breakdown characteristic of the order of a few microseconds even when shielded from the light.

References Cited in the file of this patent UNITED STATES PATENTS 1,376,604 Case May 3, 1921 1,531,301 Metzger Mar. 31, 1925 1,651,308 Winkelmann Nov. 29, 1927 1,658,568 Moore Feb. 7, 1928 1,852,808 Johnston Apr. 5, 1932 2,072,733 Dobke Mar. 2, 1937 2,194,145 Hansell Mar. 19, 1940 2,269,442 Dench Jan. 13, 1942 2,374,677 Goldstein et al. May 1, 1945 FOREIGN PATENTS 428,629 France June 26, 1911 

1. A GLOW DISCHARGE DEVICE HAVING A PRECISE TIMEBREAKDOWN CHARACTERISTIC UPON APPLICATION OF VOLTAGE THERETO, COMPRISING AN ENCLOSING ENVELOPE PERMEABLE TO RADIATION HAVING A WAVELENGTH BETWEEN ABOUT 3200 AND 4900 ANGSTROM UNITS, A PAIR OF COOPERATING ELECTRODES IN SAID ENVELOPE AT LEAST ONE OF SAID ELECTRODES BEING A COLD CATHODE HAVING AN ACTIVATING COATING OF OXIDES OF THE ALKALINE EARTH METALS, SEALING AND SUPPORTING STRUCTURE FOR SAID ELECTRODES, AN IONIZABLE MEDIUM WITHIN THE ENVELOPE COMPRISING A GAS FROM THE GROUP CONSISTING OF THE RARE GASES AND CAESIUM, MATERIAL POSITIONED EXTERNALLY OF AND CONTIGUOUS SAID ENVELOP AND PRODUCING RADIATION HAVING WAVELENGTH BETWEEN ABOUT 3200 AND 4900 ANGSTROM UNITS FOR EFFECTING PHOTO-EMISSION OF ELECTRONS FROM SAID ONE ELECTRODE, AND OXIDE COATING ON SAID STRUCTURE OF LIGHT METAL WHICH REPELS CESIUM FOR PREVENTING LEAKAGE BETWEEN SAID ELECTRODES. 